The present invention relates to a method for manufacturing helical screw rotors. The invention also relates to rotors manufactured in accordance with the method.
Normally, helical screw rotors are machined from solid metal blanks. Moreover, the rotors have a complicated geometric shape, which places a great demand on accuracy in manufacture. Machining of the rotors from solid rotor blanks also involves the removal of large quantities of material from the blank. All of this results in long manufacturing times and high production costs.
Consequently, there has long been a demand for a simpler method for the manufacture of helical screw rotors, without the need to observe such high precision in manufacture, such that rotors of this kind can be produced in large numbers at reasonable cost.
Swedish Patent Application No. 8603720-7 and PCT/SE87/00397 teach a number of known methods for the manufacture of helical screw rotors, in which manufacture is simplified by moulding a plastic surface layer onto a plastic or a metal rotor body, which in turn is mounted on a metal shaft. These known methods are based on the concept of first producing a rotor body with no particular attention to measurement accuracies, and then moulding onto the body a surface layer formed to exact measurements. The desired result is normally achieved, when the rotor body is made of metal. Although plastic-moulded rotor bodies represent a considerable simplification in the manufacture of such bodies, subsequent shrinkage of the plastics material creates drawbacks which are not found when casting or moulding other kinds of plastic objects, where subsequent shrinkage of the plastics material can be anticipated and compensated for. The particular difficulties which manifest when moulding helical screw rotors relate to the helically extending cams, which as a result of the shrinkage become distorted in a manner which cannot readily be calculated prior to manufacture. This distortion changes the pitch of the cams in the vicinity of the ends thereof.
The effect of this shrinkage-induced distortion can be eliminated in some cases, by imparting to the thin-wall lands or threads of female rotors a degree of elasticity which will allow the lands to be deformed elastically in operation. This feature will also afford certain advantages, as reported in PCT/SE86/00109.
Another solution must be found, however, in the case of male rotors and female rotors of larger land-wall thicknesses.